1. Field of the Invention
This invention relates to thermally stable polymeric substrates which exhibit piezoelectric and pyroelectric properties. In particular, it relates to thermally stable, piezoelectric and pyroelectric polymeric substrates which do not require polarization solvents or mechanical orientation to prepare them.
2. Description of the Related Art
Production of an electric voltage in response to mechanical excitation (piezoelectricity) or thermal excitation (pyroelectricity) requires a material to have a preferred dipole orientation in its structure. This preferred orientation or polarization occurs naturally in some crystals such as quartz and can be induced into some ceramic and polymeric materials by application of strong electric or mechanical fields. For some materials, a combination of mechanical and electrical orientation is necessary to completely polarize the material. Such is the case for the only commercially available piezoelectric polymer, poly(vinylidene fluoride) (PVF.sub.2).
Poly(vinylidene fluoride) is a highly crystalline material (40-60%) possessing at least three major crystal forms: .alpha.-form, .beta.-form, and .gamma.-form. Of these forms, only the .beta.-form can be polarized to exhibit significant piezoelectric or pyroelectric responses. The .beta.-form of crystallinity is induced by drawing the PVF.sub.2 sheet at a temperature of approximately 70.degree..degree.C. to several times its original length. This produces a morphology that has vinylidene fluoride groups aligned in an orthorhombic, planar zigzag conformation.
When the PVF.sub.2 is subsequently poled in a DC field of at least 100 MV/m, the molecular dipoles (negatively charged fluorine atoms and positively charged hydrogen atoms) will orient perpendicular to the plane of the film producing a net dipole in the field direction. This process is known as poling.
After poling, the crystalline phase in the PVF.sub.2 serves as a structural support for the induced polarization. It is widely considered that crystallinity is a necessary morphological feature for making a stable polarized material.
However, material and process limitations prohibit the use of PVF.sub.2 in numerous device applications, particularly where thermal stability is required. At temperatures significantly above the glass transition temperature of the non-crystalline regions of PVF.sub.2, the polarization induced in the poling process is randomized. This dipole randomization occurs around 60.degree..degree.C. and is accompanied by a tremendous loss in the induced polarization. This destroys or significantly decreases the desired piezoelectric and pyroelectric properties. Moreover, the need to mechanically draw the film prior to electrical poling presents a significant disadvantage in that drawing in the film direction causes an anisotropic elastic modulus in the stretching or axial direction as compared to the transverse direction.
Scheinbeim et al. (U.S. Pat. Nos. 4,830,795 and 4,863,648) disclose a process for making polarized material which does not require mechanical drawing. In their process, polarized material is prepared by applying an electrical field to a material in a polarization solvent. During poling the solvent is removed from the material to the desired extent or totally to provide a polarized material which is substantially free of mechanically induced orientation. This process has been used to pole PVF.sub.2 and nylon materials such as Nylon 7 and Nylon 11. However, this process is only suitable for polymers which are soluble in an appropriate polarization solvent. This process cannot be used for polymers which are not soluble in a polarization solvent.
Improper removal of the solvent from the polarization material also adversely affects the final product. If the percentage of solvent remaining in the poled material is too large, the enhanced mobility of the material may actually promote randomization of the dipolar units leading to lower desired polarization properties.
Lastly, the use of solvent dictates the conditions of the process. For example, the temperature at which the solvent is removed must be well below the degradation temperature of the polymer. In addition, the poling temperature must be lower than the boiling point of the solvent.
An object of the present invention is to provide a polymeric substrate which is thermally stable and exhibits piezoelectric and pyroelectric properties.
Another object of the invention is to provide a process for preparing a thermally stable polymeric substrate which exhibits piezoelectric and pyroelectric properties without requiring mechanical orientation.
Another object of the invention is to provide a process for preparing a thermally stable polymeric substrate which exhibits piezoelectric and pyroelectric properties without requiring the use of solvents.